The invention relates to a two-stroke engine equipped with a carburetor controlled by a throttle flap. In two-stroke engines of the above-mentioned type, the reciprocating motion of the piston subjects the crankcase to a continuous change between overpressure and underpressure. The invention is based on the realization that these pressure or oscillation relationships in the crankcase of a two-stroke engine can be made use of, especially by separating the overpressure waves from the underpressure waves in the crankcase, in order to provide a pneumatic auxiliary system for servo functions, in particular for controlling or limiting the engine speed to protect the engine or its parts from overload.